refactor(battle-node): unify IMatchingBridge.RegisterBattle signature

Single RegisterBattle(p1, p2?, type) with contract validation throws on
invalid combinations (Pvp requires both; Bot requires p2==null; Scripted
accepts either). PendingBattle carries Type + P1 + nullable P2. Handler
+ controller adapt; v1.2 behaviour preserved because Scripted is the
only type used today (Phase 2 adds Pvp, Phase 3 adds Bot).

SVSim.BattleNode/Bridge/IMatchingBridge.cs

@@ -1,12 +1,25 @@
+using SVSim.BattleNode.Sessions;
+
 namespace SVSim.BattleNode.Bridge;
 
 public interface IMatchingBridge
 {
     /// <summary>
-    /// Mint a battle id, register a pending session for the given viewer with their per-battle
-    /// MatchContext snapshot, and return the URL the client should open a socket to.
+    /// Mint a battle id, register a pending session, return the URL the client should
+    /// open a socket to.
     /// </summary>
-    PendingMatch RegisterPendingBattle(long viewerId, MatchContext context);
+    /// <remarks>
+    /// Contract rules (enforced; throws <see cref="ArgumentException"/>):
+    /// <list type="bullet">
+    ///   <item><c>Pvp</c>: <paramref name="p2"/> required. Both viewers expected to
+    ///         connect WS within 60s.</item>
+    ///   <item><c>Bot</c>: <paramref name="p2"/> must be null. One viewer expected;
+    ///         opponent runs in client.</item>
+    ///   <item><c>Scripted</c>: <paramref name="p2"/> currently null; future
+    ///         server-driven bot config rides on <paramref name="p2"/>.</item>
+    /// </list>
+    /// </remarks>
+    PendingMatch RegisterBattle(BattlePlayer p1, BattlePlayer? p2, BattleType type);
 }
 
 public sealed record PendingMatch(string BattleId, string NodeServerUrl);